1. Field of the Invention
The present invention relates to a signal extraction apparatus and method. Such an apparatus may be used to extract a measurement signal from a drive signal for a transducer.
2. Discussion of Prior Art
A known type of transducer is the so-called oscillating or resonant beam transducer which is forced into oscillation at its resonant frequency by the impingement of light which has been amplitude-modulated at the resonant frequency of the beam.
In order to provide a useful measurement signal, light of a particular optical wavelength is first amplitude modulated by drive elements at the resonant frequency of the beam and is then directed onto the beam. The resonant beam is thus caused to oscillate at its resonant frequency by the modulated "drive" light.
Another light source produces continuous or D.C. light of a different optical wavelength and this "detecting" light is directed towards the beam where it is amplitude modulated by movement of the beam. The reflected detecting light can be extracted from the reflected drive light by an optical wavelength filter because of the difference in optical wavelength. The extracted amplitude modulated detecting light is then directed onto a photodetector which provides an output representing the oscillating beam movement.
Although this arrangement works very well, it does require the presence of two light sources of different optical wavelengths which makes the arrangement somewhat expensive. Also, for convenience, a single optical fibre is used to transmit light from both sources to the oscillating beam and to return the reflected light to the photodetector. This requires the presence of two optical splitters, one being a beam splitter or Y-coupler and the other being a wavelength filter, which makes the arrangement expensive and cumbersome. The cost and complexity of this arrangement therefore precludes the use of transducer arrangements of this type in many applications which would otherwise be attractive.